It is a matter of prosecutorial discretion. Technically, those notes were government property since they were made as a government employee. The fact that Comey released them clandestinely via a law professor friend to the media with no other FBI knowledge or permission points to the fact that Comey knew what he was doing was skirting the letter of the law. If he truly felt Trump's actions criminal in nature, and he said he did this to evoke a special prosecutor, then his withholding the info for so long constitutes a felony. Comey never meant for his having to admit these actions.
Comey also said that he did not know whether Trump's actions as he understood or saw them constituted obstruction, that it was up to the hearing board--- --- but that is EXACTLY his job as a lifetime career prosecutor to know these things!!! Again, by saying so, he is trying to avoid the legal jeopardy of being charged with violation of US Code for withholding criminal evidence, a felony.
So will Comey be prosecuted? Probably not. Subject to interpretation, the government may want to keep Comey as a fallback friendly witness should he be needed again for any further testimony down the road, but that does not mean that no one will want to question him further on other matters, such as Loretta Lynch.